1. Field of the Invention
This invention relates to a method of diagnosing the presence of deterioration of a 3-phase lightning arrester assembly which includes one or more series-connected non-linear resistance elements comprised of zinc oxide, enclosed within a container and forming respective phase arresters of the 3-phase lightning arrester assembly. The method uses resistance currents in phase with terminal voltages of the arresters which have passed through the arresters.
2. Description of the Related Art
In a conventional lightning arrester assembly comprising one or more series-connected non-linear resistance elements comprised of zinc oxide and enclosed in a container, the currents which can flow through the elements under a normal voltage are very small, usually on the order of ten .mu.A, depending upon the characteristics of the respective resistance elements. Currents having such magnitude do not raise the normal temperature of the resistance elements and, therefore, do not cause deterioration of the elements due to a raised temperature. A conventional lightning arrester assembly, which usually has no discharge gaps, can be placed in series with the corresponding non-linear resistance elements and can use currents of a small magnitude without being deteriorated.
However, the non-linear resistance elements may be deteriorated due to thermal cycles, based on the operational and atmospheric conditions under frequently-occurring abnormal voltages. And, if this deterioration advances, the elements will not be able to withstand the normal phase voltages in the power system and may break down, thereby hindering the system operation. Therefore, it is desirable to provide a method of continuously monitoring the resistance currents during the operation of the arrester assembly in order to determine the initial deterioration of the non-linear resistance elements.
FIG. 15 illustrates the voltage-current characteristics of a typical non-linear resistance element. In FIG. 15, the solid line 25 shows the characteristic of a normal resistance element and the alternate long and short dashed line 35 illustrates the characteristic of a deteriorated resistance element. The current represented by the I-axis shows a resistance current which does not contain any effects based on the electrostatic capacitance between both end faces of a disc-type non-linear resistance element. A phase voltage in the system is designated as Vn. The current flowing through a resistance element under such phase voltage varies depending upon the temperature of the element, and if the element is deteriorated, the current varies greatly even at the same temperature. For example, if the temperature of the element initially is .theta..sub.2, when the current I.sub.R1 flows through the element when the element is functioning normally, the current varies greatly until it becomes equal to I.sub.R2. Therefore, by measuring the current flowing normally and comparing this measured current with the current flowing through the normal resistance element corresponding to the temperature of the resistance element when the measurement is performed, the presence of deterioration can be accurately determined. In FIG. 15, the temperature relationship .theta..sub.2 &gt;.theta..sub.1 holds.
If the current flowing through an arrester is measured by a current sensor connected in series with the arrester, the sensed current contains a resistance current as well as a capacitive current because the voltage applied to the arrester includes an alternating current having a running frequency.
FIG. 16 shows a lightning arrester 2 connected across a power line and to ground. If the arrester 2 has insulative container enclosing the resistance elements, it is considered as containing the insulator and the resistance elements, but if the container is made of metal it is grounded and treated as containing the resistance elements alone.
FIG. 17 shows an electrical equivalent circuit of the arrester shown in FIG. 16. Each of the resistance elements are usually formed in a disc which has a relatively large electrostatic capacitance C between the end faces thereof, so that the current flowing through the arrester includes the vector sum of a resistance current I.sub.R, raising the temperature of the element, and a capacitive current I.sub.c flowing through the capacitance. In order to determine the deterioration of a resistance element, it is necessary to extract only the resistance current I.sub.R from the vector sum. In FIG. 17, reference character L represents the inductance of the grounded conductor of the arrester. It should also be noted that when the container enclosing the arrester is made of insulative material, the capacitive current I.sub.c includes a capacitive effect due to the electrostatic capacitance of the insulator.
FIG. 18 illustrates a conventional method of measuring a resistance current and FIG. 19 illustrates the waveforms of the voltage and current measured using the method of FIG. 18. The arrester 2 connected to a bus or transmission line 1 is grounded via a current sensor 4 and the bus or transmission line is grounded via a voltage sensor 3. The outputs from the current and voltage sensors 4 and 3 are input, via amplifiers 6 and 7, to a computer 8 to compute the resistance current. The calculation of this resistance current is performed by differentiating the voltage V, measured by the voltage sensor 3; amplifying the differentiated waveforms so that the crest value of the waveform coincides with the crest value of I.sub.c of FIG. 17; and subtracting the amplified waveform from the overall current I.sub.s measured by the current sensor 4, as shown in FIG. 19. The resulting resistance current I.sub.R is compared with the normal resistance current at the temperature of the resistance element measured in parallel with this measurement to determine the presence of deterioration.
If the automatic monitoring of a 3-phase lightning arrester connected to a 3-phase transmission line is desired for use in diagnosing the deterioration of the 3-phase arrester using the above method, current sensors 41, 42 and 43 and voltage sensors 31, 32 and 33 for 3 phases are required, as shown in FIG. 20, even if the amplifiers 6 and 7 and the computer 8 used in FIG. 19 may be shared for this purpose. In addition, a changeover device 5, having many changeover contacts, is required to allow the sensed current and voltage values having the same phase to be input to respective amplifiers 6 and 7. The deterioration of the resistance elements advances at a maximum speed on the order of an hour, so that a satisfactory determination of deterioration can be performed by switching the respective phases sequentially.
As described above, the diagnosis of the deterioration of the resistance elements using the conventional method requires more current sensors and leads as the number of transmission lines increases (the voltage sensors may be shared). Furthermore, the changeover device is quite large and the use of this device complicates the diagnostic procedure. thus, a more simplified diagnostic method of desired.